Someone please correct me if I'm wrong, but I don't believe hi-flow cats or test pipes automatically legally void an auto warranty. Any aftermarket part has the possibility of voiding your warranty if the part in question directly causes a malfunction. The dealership can refuse to cover the repair under warranty since it was not a stock manufacturer part. Even though test pipes are not road legal (at least here), I don't think that legally voids a warranty for say your clutch, headlights, etc. Like I said, someone feel free to correct me if I'm wrong.

How much louder would test-pipes make an 18" resonated FI exhaust?

Got my gen 3 first then my non res cf FI exhaust with stock cats. IMHO, most amazing sound. Just loud enough and it sounds absolutely amazing. Zero regrets! I almost always drive with the windows down just to hear the exhaust. I have a few videos on YouTube with a GoPro mounted right on the bumper above the exhaust if you want to get an idea. (RoyaltyB)

IMO skip the 12 inch res and go non res with the stock cats.

thank you man, I'm deff getting the non res cbe , i might also go along with the carbon fiber. i will deff check out your videos

cabin noise , will i still be able to listen to music comfortably ?

After installing my FI, my aftermarket audio has become a waste of money.

Exhaust note > Music.

How much louder is the FI with 18" resonators compared to the stock NISMO exhaust?

good video for non res, stock cats...

https://www.youtube.com/watch?v=CmiH8gzjGeo

I have the FI w/ 12" resonators and FI LTH on my 09 nismo and it is night and day from stock. Like a different car almost...HR engine on steroids. The volume is not headache-inducing nor is the exhaust note super different from stock, just really enhances the growl like the pipes are up to a megaphone. There is the typical hiss/buzz at cruising and lower rpm and a lot of cabin drone. But imPo there is no better full exhaust. If you're on the fence I say definitely go for FI!!!! Full disclosure, I didn't personally install it but the previous owner did (and babied her - thank you ryanboy) which is why I bought her. I'm in love and from what I can tell all other FI customers are very happy as well.